The quantitative aspects of in vivo metabolic fuel transport is being studied in children with infusions of deuterium and carbon-13 labelled glucose, alanine, and palmitic acid, and with nitrogen-15 labelled lysine. Isotopic enrichment is measured in appropriate, microliter-sized plasma samples by combined gas chromatography-mass spectrometry with specific ion monitoring techniques using a PDP-12 computer-controlled multiple ion monitoring system interfaced with an LKB-9000 GC-MS or by an accelerating voltage alternation-voltage sweeping circuit designed for an AEI-MS12 GC-MS. These studies have re- affirmed the safety of stable isotopic techniques in childhood and are directed to defining abnormalities of metabolic fuel in infants and children whose fuel balance is precarious in various clinical situations.